With the advent of internet-connected devices and the digital health industry, health and wellness monitoring has become an area of growing focus. Monitoring vital signs such as heart rate, ballistocardiogram signals, and breathing rate is desirable both inside and outside healthcare facilities. Within healthcare settings, vital sign tracking can be essential for: ensuring patient safety when a healthcare provider is not present at a bedside, diagnosing medical conditions, monitoring a patient's progress, and planning a patient's care. Outside of healthcare settings, tracking vital signs and posture enables individuals to quantify and conceptualize their health status, thereby helping individuals remain mindful of their health and wellness needs, visualize progress, and maintain the motivation needed to achieve health and fitness goals.
Current vital sign trackers in the consumer market are fairly intrusive, for example, current heart rate monitors often require an individual to strap the monitor around the individual's chest. Many vital sign trackers include just one type of sensor configured to detect one type of vital sign, such as for example, heart rate. Additionally, many vital sign monitors in the consumer market are not very accurate. In the healthcare setting, much more accurate devices are available, but they are often very large devices positionable at a patient's bedside, requiring a connection to an electrical outlet and leads attached to the patient. Attachment to these bedside devices can cause anxiety in patients, and the devices are expensive, not portable and prone to electromagnetic interference (EMI).
Optical fiber sensors have gained increased attention in the research setting, as an alternative to existing vital sign monitors. Optical fiber sensors are chemically inert and resistant to EMI. Moreover, they can be portable and integrated into fixtures, such as mattress pads and cushions. Fixture-integrated devices have numerous advantages over bedside appliances and wearable instruments. For example, fixture-integrated devices allow for a reduction in loose connecting wires or wireless data transmitters between sensors, electronics, and power supplies. This reduction may lead to increased reliability, data quality, and security.
However, optical fiber sensors developed to date have not proven to be suitable alternatives to conventional monitoring systems. For example, in “Optical Fibre Sensors Embedded into Medical Textiles for Healthcare Monitoring,” IEEE Sensor J. 8 (7), 1215-1222, 2008, Grillet el at proposed integrating a single mode macro-bending fiber sensor into a belt to measure respiratory rate. A macro-bending sensor typically experiences significant light loss due to macroscopic deviations in the fiber's axis from a straight line, resulting in low sensitivity. Such a sensor would be unlikely to detect the subtle movements of the chest wall needed to accurately measure heart rate or ballistocardiogram signals.
In an effort to improve sensitivity, others have proposed alternative approaches for fiber optic sensors. For example, U.S. Pat. No. 6,498,652, Varshneya et al. disclosed a fiber optic monitor that utilizes optical phase interferometry to monitor a patient's vital signs. Optical phase interferometry has several limitations. For example, while Fabry-Perot interferometric sensors and Mach-Zehnder interferometric sensors are sensitive to mechanical vibrations of the body, they are also highly sensitive to mechanical vibrations external to the body, as well as temperature, acoustic waves, magnetic fields, and other environmental, noise. Thus, without proper equipment, interferometer sensors are not suitable for monitoring vital signs due to unreliable performance caused by signal fading and inaccuracies resulting from environmental noise-induced phase change. The equipment needed to filter out the environmental noise includes an expensive phase modulator and coherent optical sources, which add significant cost and complexity and make such sensors impractical for widespread commercial adoption. Other proposed designs have also struggled to balance sensitivity, accuracy, and cost.
Moreover, most fiber optic vital sign sensors being developed are limited to detecting heart rate, breathing rate, and/or macro-movements indicative of changes in body position. A major limitation of many of these sensors is the inability to obtain the highly sensitive ballistocardiography (BCG) waveforms. BCG is a technique used to record vibrations of the body resulting from mechanical activity of the heart. In particular, BCG measures mass movements of the heart and circulating blood generated by forces associated with heart contractions during the cardiac cycle. Historically, BCG waveforms were acquired using an extremely large, suspended table configured to support a patient lying thereon; such a suspended table was heavy, non-portable, and required substantial mechanical maintenance. Due to the cumbersome system required, BCG did not get much attention or use during much of the twentieth century; however, reliable BCG waveforms can provide significant insights into a patient's cardiac health. In addition to revealing a patient's heartbeat unobtrusively, in real-time, BCG waveforms are useful in determining heart rate variability, which is an indicator of stress on a body. Moreover, comparison of BCG and EEG waveforms, in particular, detection of the timing between the R peak of the EEG waveform and the J peak of the BCG waveform reveals beat-to-beat blood pressure changes. Additionally, as described, for example, in E. Pinheiro et al., “Theory and Developments in an Unobtrusive Cardiovascular System Representation; Ballistocardiography,” The Open Biomedical Engineering Journal, 2010, 4, pp. 201-216, the contents of which is herein incorporated by reference in its entirety, features of BCG waveforms have been found to correlate to, and suggest the presence of, a number of maladies. For example, abnormal BCG waveforms are obtained in individuals having angina pectoris, asymptomatic coronary artery disease, acute myocardial infarction, hypertension, coarctation of the aorta, and mitral stenosis, to name a few.
Despite the clinical value of monitoring BCG, it is not conventionally monitored in a healthcare setting, due to a lack of a suitable detection system. Detecting BCG waveforms requires a level of sensitivity and precision that current sensor designs are lacking. Therefore, a need exists for a physiological parameter monitoring device capable of reliably detecting BCG waveforms. A need also exists for a method of detecting vital signs, including BCG waveforms, which overcomes the limitations of existing methods. Thus, there is a need for new and useful optical fiber vital sign sensors and related methods of use.